This application is a 35 U.S.C. sec. 371 of PCT Application Ser. No. PCT/FR99/02385, filed on Oct. 6, 1999.
The present invention relates in general to liquid stirrers, and more particularly to stirrers with magnetic coupling.
Stirrers with magnetic coupling generally transmit rotary motion without contact from a driving magnetic portion to a driven magnetic portion. The driving and driven portions are disposed respectively outside and at the bottom of a receptacle of liquid to be stirred. This transmission of motion can be described as xe2x80x9ccoupling of the axial type without guidance for the driven portionxe2x80x9d.
Compared with less-recent stirrers having mechanical coupling, transmitting rotary motion without contact makes it possible to avoid having passages through the walls via mechanical rotary gaskets which present a risk of leakage.
FIGS. 1A and 1B show two known stirrers with magnetic coupling.
The stirrer of FIG. 1A comprises a driving magnetic portion 1 which can be rotated about an axis by a motor 2, in particular via transmission means 3, and a driven magnetic portion 4. The driven portion 4 is placed on the bottom of a receptacle 5 which is supported by means of a support 6 over the driving portion 1. The receptacle 5 and the support means 6 are made of non-magnetic material. The driving and driven portions 1 and 4, the transmission means 3, and the motor 2 are centered on a vertical axis of symmetry 8. The driven portion 4 is typically a permanent magnet in the form of a bar having a pair of north and south magnetic poles. The driving portion 1 is constituted by a U-shaped permanent magnet whose magnetic poles face the magnetic poles of the bar 4 when the stirrer is at rest. The magnets 1 and 4 are generally made of materials such as ferrites or alnicos (aluminum, nickel, cobalt).
When the motor 2 is in operation, the driven portion 4 is rotated about the axis of symmetry 8 by magnetic coupling with the driving portion 1 via an air gap 7. More precisely, when the driving portion 1 rotates about the axis 8 under drive from the motor 2, torque is transmitted to the driven portion 4, thereby causing it to turn about the same axis.
The receptacle 5 contains chemical reagents in liquid form. Rotation of the bar magnet 4 about the axis enables the liquids contained in the receptacle 5 to be stirred and, for example, favors the production of precipitates which are evacuated via an evacuation outlet 50 on a side face of the receptacle 5. With such a precipitation reaction, the stirrer shown in FIG. 1A can be referred to as axe2x80x9cprecipitatorxe2x80x9d.
FIG. 1B shows another known stirrer. In FIG. 1B, those elements which are identical to elements of FIG. 1A are designated using the same references. The stirrer of FIG. 1B differs from the stirrer shown in FIG. 1A in that the driving portion 1, the motor 2, and the transmission means 3 are replaced by a static drive 9 powered by an alternating (square wave) voltage source 10. The static drive 9 has vertically disposed electromagnets (one of which is shown diagrammatically in FIG. 2B) which are powered by a voltage delivered by the source 10 and are switched in alternation. The static drive 9 produces the same effect as the elements 1, 2, and 3 in FIG. 1A, i.e. it produces a rotating magnetic field which causes the bar magnet 4 to rotate by magnetic coupling about a vertical axis of symmetry 8xe2x80x2.
The stirrer with static drive shown in FIG. 1B presents several advantages over the stirrer with rotary motor of FIG. 1A. In particular, it does not require mechanical moving parts to be used and it is more compact. In addition, with the stirrer with static drive it is possible to vary the torque transmitted to the driven portion merely by modifying the amplitude of the current delivered to the coils of the electromagnets. In the case of the stirrer with rotary motor, the transmitted torque can be adjusted only by physically varying the size of the air gap by means of a mechanical device.
A major drawback of prior art stirrers as shown in FIGS. 1A and 1B lies in the fact that the amount of torque that can be transmitted to the driven portion is limited. Increasing this torque increases the attraction force pulling the bar 4 against the bottom of the receptacle and increases wear by friction both of the bar and of the bottom of the receptacle.
FIGS. 2A and 2B are diagrammatic front views of the relative positions of the driving and driven portions when the driving portion is constituted by a permanent magnet 1 (FIG. 2A) and when it is constituted by electromagnets 9 (FIG. 2B). FIG. 2C is a diagrammatic plan view showing the arrangement of FIG. 2A. As shown in FIG. 2C, when the stirrer is in operation, the driven portion 4 continuously lags behind the rotary field produced by the driving portion by an angle xcex1. The field lines, represented by arrows 11 (FIGS. 2A and 2C) and 12 (FIG. 2B) between the poles of the driving and driven portions have a horizontal component (FIG. 2C) which contributes to the torque transmitted to the driven portion, and a vertical axial component (FIGS. 2A and 2B) parallel to the axis of rotation 8, 8xe2x80x2. The axial force due to the axial component constitutes a very large fraction of the interaction energy between the driving and driven portions. Any increase of the torque transmitted to the driven magnet 4 automatically increases the axial attraction between the driving and driven portions and thus increases wear of the driven magnet 4 and of the bottom of the receptacle 5 because of the presence of a significant axial component in the field lines.
The present invention seeks to provide a liquid stirrer which, compared with prior stirrers, makes it possible to reduce the wear on the driven portion and the bottom of the receptacle for given torque transmitted to the driven portion.
To this end, the invention provides a liquid stirrer with magnetic coupling comprising a driving portion and a driven portion, the driven portion being for placing on the bottom of a receptacle containing a liquid to be stirred, and control means for controlling the driving portion so as to drive the driven portion in rotation about a predetermined axis of rotation by means of magnetic coupling with the driving portion, the stirrer being characterized in that the driving and driven portions are configured so as to encourage the field lines that result from the magnetic coupling to extend substantially perpendicularly to said axis of rotation in the vicinity of the driven portion.
In practice, the driving and driven portions are preferably configured so as to encourage the field lines resulting from the magnetic coupling to extend substantially parallel to a longitudinal axis of the driven portion in the vicinity of the driven portion.
The predetermined axis of rotation is typically, but not necessarily, a vertical (virtual) axis of symmetry of the driven portion and/or of the driving portion. When the stirrer is in operation, the driven portion is merely placed on the bottom of the receptacle and is therefore subject only to its own weight, to friction forces with the bottom of the receptacle, and to the electromagnetic forces generated by the driving portion through the receptacle. The receptacle, or at least a portion thereof close to the driving portion, is made of a non-magnetic material, so as to allow the field lines to pass through.
Thus, contrary to prior stirrers, in the stirrer of the invention, a large part of the field lines in the vicinity of the driven portion have an axial component (parallel to the axis of rotation) which is small compared to their horizontal component. The axial attraction force which is an undesirable force because of the wear phenomena to which it gives rise both on the driven portion and on the bottom of the receptacle, is therefore smaller for identical torque. Consequently, a larger torque can be transmitted to the driven portion without increasing its wear, and without increasing wear on the bottom of the receptacle. Experimentally, the present inventors have observed that under certain conditions it is possible to increase torque by about 30%.
The increase in torque made possible by the present invention makes it possible to obtain greater stirring capacity, and, for example, to deal with the bottom of the receptacle becoming caked and with possible variations in the viscosity of the liquids contained in the receptacle. It also makes it possible to increase the air gap between the driving and driven portions, e.g. to allow the use of receptacles that are of greater thickness.
In the invention, the driven portion comprises a permanent magnet which is preferably made of neodium-iron-boron or of samarium-cobalt. These materials withstand demagnetization very well, unlike the materials conventionally used in prior stirrers which have a tendency to become demagnetized quite easily under the effect of an opposing magnetic field, which means that the magnets need to be replaced regularly, thereby increasing maintenance costs of such stirrers.
In a first embodiment of the present invention, the driving portion comprises at least one permanent magnet having at least one pair of magnetic poles whose active faces are substantially parallel to the axis of rotation. The magnetic poles of a given pair are typically of opposite polarities.
Advantageously, at least the poles of the permanent magnet of the driving portion are made of an anisotropic material. The permanent magnet is placed in such a manner that the magnetization direction of the anisotropic material is substantially perpendicular to the axis of rotation of the driven portion. The anisotropic material comprises, for example, strontium ferrite.
The magnetic poles of the permanent magnet of the driving portion are optionally united by a central part made of a ferromagnetic material such as soft iron. This central part makes it possible to avoid magnetic leakage in a direction parallel to the axis of rotation between the poles of the permanent magnet.
The control means comprises drive means for rotating the driving portion, which drive means includes a motor and transmission means for coupling the motor to the driving portion.
In a second embodiment of the invention, the driving portion comprises at least one electromagnet having at least two pairs of magnetic poles whose active faces are substantially parallel to the axis of rotation. The control means then comprises power supply means for feeding said at least one electromagnet with AC.
Said at least one electromagnet is typically constituted by an integer number p of electromagnets where p is greater than or equal to 2, and the power supply means feeds said p electromagnets with p-phase AC. The p electromagnets are arranged in a cross, each electromagnet constituting one of the branches of the cross.
In a third embodiment of the present invention, the driving portion comprises at least one permanent magnet having at least one pair of magnetic poles, and the distance between the magnetic poles of a given pair is substantially equal to or greater than the size of the driven portion in any direction perpendicular to the axis of rotation. The term xe2x80x9csubstantially equalxe2x80x9d is used to mean a distance which is equal to, slightly greater than, or slightly less than the size of the driven portion in any direction perpendicular to the axis of rotation. The distance between the magnetic poles of a given pair is measured between the facing internal faces of the pair of poles.
The active faces of said at least one pair of magnetic poles are preferably substantially perpendicular to the axis of rotation.
The driving portion is rotated by drive means constituted by a motor and transmission means for coupling the motor to the driving portion.
In a fourth embodiment of the invention, the driving portion comprises at least one electromagnet having at least two pairs of magnetic poles, and the distance between the magnetic poles of a given pair is substantially equal to or greater than the size of the driven portion in any direction perpendicular to the axis of rotation. The control means comprises power supply means for feeding said at least one electromagnet with AC. Said at least one electromagnet is constituted, for example, by an integer number p of electromagnets where p is greater than or equal to 2, and the power supply means feeds said p electromagnets with p-phase AC. The driving portion comprises a yoke having p pairs of teeth extending substantially parallel to the axis of rotation, each pair of teeth constituting cores having coils of an electromagnet wound thereabout. The yoke is made up of laminations that are generally cylindrical in shape, concentric, and stacked radially.
In the above embodiments, the driven portion may be in the form of a bar having at least a central portion that is cylindrical. In addition, a magnetic element capable of producing asymmetry in the field lines can be provided so that the driven portion is also rotated about a longitudinal axis thereof, perpendicular to said axis of rotation, by magnetic coupling. This magnetic element can be situated on the driven portion or on the driving portion, and more precisely it can be situated on or close to one of the magnetic poles of the driving portion or of the driven portion. The magnetic element is a part made of a ferromagnetic material such as soft iron.
In another aspect of the invention, the stirrer includes a confinement wall made of a non-magnetic material in particular for confining the receptacle and protecting the driving portion from the liquid contained in the receptacle. The stirrer can thus be used to stir reagents that are dangerous, e.g. including nuclear materials.